Recently, a conventional mechanical meter installed on an instrument panel of a vehicle has been replaced with an image display apparatus such as a liquid crystal display (LCD) apparatus. One advantage of such an image display apparatus is that design and layout of a screen image can be freely adjusted. As disclosed, for example, in JP-A-2004-157434, an image display apparatus has been proposed that displays a navigation image of a navigation system, in addition to a meter image corresponding to a mechanical meter.
In a color LCD used in such an image display apparatus disclosed in JP-A-2004-157434, brightness of a screen is adjusted by adjusting luminosity of light emitted by a backlight device. Specifically, as shown in FIG. 12, the brightness of the screen is adjusted by adjusting a duty ratio of a pulse width modulated (PWM) voltage applied to the backlight device. For example, when the image display apparatus operates in a day mode, the duty ratio of the PWM voltage is set to 100% so that the brightness of the screen is set to a maximum level (i.e., level 5). When the image display apparatus operates in a night mode, the duty ratio of the PWM voltage is set to 60% so that the brightness of the screen is set to a medium level (i.e., level 3).
Likewise, in a self-light-emitting display apparatus (e.g., organic electroluminescence (OEL) display) constructed with self-light-emitting elements, brightness of a screen is adjusted by adjusting voltage (or present) applied to the elements.
In the conventional image display apparatus described above, brightness of an image displayed on a screen varies with brightness of the screen. Therefore, as the brightness of the screen is set to a lower level, the image displayed on the screen becomes less-visible to an user (i.e., driver and occupants). As a result, even when a warning image indicating abnormal condition of a vehicle is displayed on the screen, the user may not find the warning image immediately.